The invention relates to a method and to a device for determining the soot charge in a combustion chamber during operation.
In the past there has been a continuous effort towards improving the process of burning fossil fuel in a combustion chamber. This holds not only for gaseous pollutants such as carbon monoxide and nitrogen oxides, but also for solid pollutants, such as soot, in the exhaust gas. In order to achieve an optimized combustion process, the firing must be optimized by using a suitable firing control. Specifically, when fossil fuel or refuse is burned, there are fluctuations in the calorific value of the fuel or the refuse mixture because of the different origin of the fuel or of the heterogeneous composition of the refuse. Moreover, when using fuel mixtures, a ratio of the individual fuels in the fuel mixture can fluctuate.
One possibility of optimization is to determine the soot charge or soot load during operation, the soot charge determined subsequently being used to control the flame. A known procedure uses a punctiform extraction of exhaust gases having soot fractions with the aid of an extracting probe. The extraction can be performed either in the combustion chamber or in a downstream exhaust gas system. Subsequently, the extracted air quantity is examined and the soot charge is determined thereby. It is impossible to provide a complete detection of the soot charge with this procedure, since extraction is performed only in a punctiform fashion. Local fluctuations in the soot charge in the combustion chamber or in the exhaust gas system therefore lead to a distorted detection result. Moreover, the soot charge produced during combustion is not detected until after a certain delay time. The firing control provided therefore always operates with a comparatively large dead time, which can be up to a few minutes in the case of relatively large power plants.
Another approach determines the soot charge of flames with the aid of laser absorption measurements using the Mie theory. This measuring method is, however, suitable only for research purposes in the laboratory, since the measurement of the soot charge in a flame is very complicated. It is currently impossible to use it in daily continuous operation.
U.S. Pat. No. 5,797,736 describes a method and a device through the use of which formation of a flame in a combustion process is controlled. In this case, sensors are used which detect characteristic parameters of combustion in and near the flame such as, for example, the temperature, the distribution of particles etc.
However, it is not stated in more detail in U.S. Pat. No. 5,797,736 how, for example, a soot charge in a combustion chamber is determined quantitatively. The main goal is to detect in the flame qualitative irregularitiesxe2x80x94such as, for example, fluctuations in the temperature distribution of the flame (xe2x80x9ccooler spotsxe2x80x9d) which can be caused, inter alia, by an irregular distribution of combustion particlesxe2x80x94in order to align a configuration of beam reflectors such that radiant energy of the flame is reflected specifically onto xe2x80x9ccooler spotsxe2x80x9d inside the flame. This renders the combustion more uniform.
A quantitative determination of the soot charge in a combustion chamber is not possible with the aid of the last-named method and device.
It is accordingly an object of the invention to provide a method and a device for determining a soot charge which overcome the above-mentioned disadvantages of the heretofore-known methods and devices of this general type and which allow a quick and simple determination of the soot charge in a combustion chamber during operation.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for determining a soot charge in a combustion chamber, the method includes the steps of:
measuring a spatial distribution of at least one parameter characteristic of a combustion by monitoring a flame in the combustion chamber, the at least one parameter allowing a conclusion concerning a soot charge in the combustion chamber during operation and the at least one parameter being at least one of a temperature and a carbon monoxide content; and
determining the soot charge based on the measuring step and by using a comparison with given conversion curves.
In other words, the object of the invention is achieved by virtue of the fact that the spatial distribution of the temperature, and/or the content of carbon monoxide is measured as a parameter characteristic of the combustion, and the soot charge or soot load is determined by comparison with given conversion curves.
Such conversion curves are available in printed form for various fuels, for example in the xe2x80x9cVDI-Wxc3xa4rmeatlasxe2x80x9d [xe2x80x9cVDI Heat Atlasxe2x80x9d] and in xe2x80x9cTechnische Verbrennungxe2x80x9d [xe2x80x9cTechnical Combustionxe2x80x9d], Warnatz, Springer-Verlag [Warnatz, Springer-Publisher]. Alternatively, or in addition, these conversion curves can be determined by experiments on different fuels or fuel compounds and stored in the form of a characteristic diagram.
The spatial distribution of the temperature can be detected by one or more suitable sensors. The measurement is accurate and contactless and requires no moving parts and is performed without delay. The content of carbon monoxide is measured, for example, by detecting the radiation in the radiation region characteristic of carbon monoxide. This radiation region is, for example, isolated from the overall spectrum of the flame by a beam splitter and subsequently detected; a suitable evaluation unit for the spatial distribution of the carbon monoxide is, for example, a CCD (charge-coupled device) camera.
The invention proposes to replace the previously known direct method for determining the soot charge or soot load by an indirect method. It is possible to avoid extracting soot-charged exhaust gases or an expensive direct determination of the soot charge in the flame. Rather, a simple measurement is used to detect parameters characteristic of a combustion, and the soot charge is determined subsequently on the basis of this measurement and on comparison with given conversion curves. There is no need for expensive extraction and analysis devices. Furthermore, the determination of the soot charge is performed according to the invention without a time delay, and thus an optimum firing control can be achieved.
By comparison with a one-dimensional measurement, more accurate determination of the soot charge is possible by measuring the spatial distribution of the temperature and/or the content of carbon monoxide, since both variables are generally not constant in the region of the flame.
Furthermore, in the case of turbulent combustion which is always present in the combustion chamber of power plants, the position of the flame changes during combustion. A stationary measurement at individual selected points could lead to the flame not being detected by the measuring device when varying its position. This can be prevented when detecting the spatial distribution by prescribing a spatial measuring zone.
In an advantageous embodiment it is possible to prescribe for the measured values a permissible range with a lower limit and/or upper limit. If a measured values lies outside the prescribed range, it cannot be taken into account when determining the soot charge. A lower limit of, for example, 800xc2x0 C. can, for example, be set when measuring the temperature. Ranges in which the temperature lies below this limit can then be regarded as lying outside the flame and be left out of account when determining the soot charge.
In an advantageous embodiment, the local soot formation rate is determined based on the measured spatial distribution of the temperature and/or the content of carbon monoxide. This means that the local formation rate associated with one or more discrete locations inside the spatial measuring zone is determined from the discrete measured values, associated with the discrete location, of the temperature and/or the content of carbon monoxide, the discrete measured values, associated with the discrete location, of the temperature and/or the content of carbon monoxide being taken from the spatial distribution of the measured values. The measuring accuracy is improved thereby.
Advantageously, the local soot formation rate is calculated using physical and/or chemical relationships. The local soot formation rate can be determined thereby without prior tests and empirical values by prescribing the fuel or the fuel mixture.
The determined soot formation rate is advantageously summed over the measuring zone. This reduces the data volume to be processed. At the same time, a total value for the soot formation rate results which can already be used for monitoring and control purposes.
In accordance with an advantageous embodiment, the determined soot formation rate is summed over a prescribeable time interval. Fluctuations in the flame, in particular due to turbulent combustion, can be reliably detected. Peak values or minimum values are simultaneously smoothed. Moreover, the flame can be monitored or controlled by the summing. If the flame extinguishes, the soot formation rate drops drastically for a relatively long time interval. Short-term flickering is smoothed by the summing over the prescribeable time interval, while extinction of the flame leads to a permanent drop in the soot formation rate which can be detected by the method according to the invention. It is therefore also possible to monitor the flame in addition to determining the soot charge.
In an advantageous embodiment, the prescribeable time interval is variable. In particular, this time interval can be varied as a function of preceding measurements. Furthermore, the prescribeable time interval can be selected otherwise than in unchanging continuous operation when starting up or in the case of load fluctuations.
The determined soot formation rate is advantageously averaged after the summing. This averaging permits the soot formation rate to be represented with reference to the magnitude of the measuring zone such that a plurality of flames or combustion chambers of different size can be compared with one another.
In accordance with an advantageous embodiment, the determined soot formation rate is, before or after the summing, linked to a calibration factor determining the soot charge. This calibration factor permits the inference from the soot formation rate to the soot charge, and is determined in a fashion specific to the boundary.
The calibration factor is advantageously variable, in particular as a function of the measured value, the combustion air fed to the flame and/or other parameters. Adaptation to different boundary conditions is thereby achieved.
It is advantageous for both the temperature and the carbon monoxide content to be measured and linked to one another. This procedure permits the soot charge to be determined on the basis of two different measured values, and thus permits a control. The accuracy is raised at the same time.
With the objects of the invention in view there is also provided, a device for determining a soot charge in a combustion chamber, including:
at least one sensor for measuring a spatial distribution of at least one parameter characteristic of a combustion by monitoring a flame in a combustion chamber, the at least one parameter allowing a conclusion concerning a soot charge in the combustion chamber and the at least one parameter being a temperature and/or a carbon monoxide content; and
a data processing device operatively connected to the at least one sensor, the data processing device determining a soot formation rate based on the spatial distribution of the at least one parameter and based on a comparison with given conversion curves.
In other words, according to the invention, a device for carrying out the method has at least one sensor for measuring the spatial distribution of the temperature and/or the content of carbon monoxide, and a data processing device for determining the soot formation rate. The data processing device includes, in particular, suitable subassemblies or modules for summing and averaging the soot formation rate and for linking with the calibration factor.
At least one sensor is advantageously configured as a CCD camera. Such xe2x80x9ccharge-coupled devicexe2x80x9d cameras permit local resolution of the measuring zone, and thus detection of the at least one parameter characteristic of combustion, in a spatial distribution.
The determined soot formation rate can subsequently be processed further via a suitable controller and provided to the flame burner.
With the objects of the invention in view there is also provided, in combination with a combustion chamber having a soot charge during operation, a device for determining the soot charge, including:
at least one sensor configured for measuring a spatial distribution of a temperature and/or a carbon monoxide content in the combustion chamber by monitoring a flame in the combustion chamber; and
a data processing device operatively connected to the at least one sensor, the data processing device determining a soot formation rate based on the spatial distribution of the temperature and/or carbon monoxide content and based on a comparison with given conversion curves.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a device for determining the soot charge in a combustion chamber, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.